Earth's moon is held in orbit by an attractive gravitational force between Earth and the moon. Tides on Earth are due mainly to the gravitational pull of the moon on Earth. Any two masses whether or not Earth and the moon, experience a mutual gravitational force that tends to pull them together. A mass in a position to be pulled to another position by a gravitational force has gravitational potential energy. Anything—water, a book, a parachutist, a molecule in the atmosphere, etc.—has gravitational energy if it is in a position to move closer to the center of Earth. Ordinarily, something has to do work to get the object to the elevated position. A book on the floor of a room has no gravitational energy if it cannot move lower than the floor. But if you lift the book, and place it on top of a table, it has gravitational energy because it is in a position to fall to the floor. The act of lifting the book and doing work produces the gravitational energy of the book. Water atop a dam has gravitational potential energy, but the water did not get into its position at the top of a dam without some agent doing work. The mechanism elevating the water could be a mechanical pump or the natural processes of water evaporating, rising in the atmosphere, condensing to liquid, and falling as rain. Ocean water pulled into a natural basin by the gravitational pulls of the moon and the sun has gravitational potential energy that we refer to as tidal energy.
Electric energy produced by a hydroelectric power plant is derived from gravitational energy. The process starts with gravitational forces pulling the water to the bottom of the dam, where the gravitational energy is converted to kinetic energy (energy due to matter in motion). When the moving water impinges on the blades of a turbine, some of the kinetic energy is converted to rotational energy, causing the turbine to rotate. The turbine is coupled to an electric generator, where the rotational energy of the turbine is converted to electric energy. The electric generator is connected to transmission lines that deliver the electric energy to consumers.
Electricity supplied to consumers is produced primarily by massive generators driven by steam turbines. The system is most efficient and most economical when the generators produce electricity at a constant rate. However, the problem is that demand varies widely by time of day. At night, when consumer demand is low, it would be seem that a utility could keep their most efficient generators running and store electric energy for times when needed. Unfortunately, there is no practical way of storing the quantities of electric energy produced by a large generator. It is possible, however, to convert the electric energy to some other form and store it. Then when electricity is neededm the secondary form of energy is converted back again to electricity. One way of doing this is to keep efficient generators running at night, when the demand is low, and use them to operate electrically driven pumps that store the water as gravitational energy in an elevated reservoir. When consumer demand picks up during the day, the water is allowed to flow to a lower level to a hydroelectric unit, where the gravitational energy is converted to electric energy. A system like this is called a pumped-storage facility. The electric energy recovered in the hydroelectric unit is always less than the electric energy used to store the water as gravitational energy. However, the former is economical because electricity produced at night is relatively cheap. There are a number of these systems throughout the world, the most prominent in the United States being the Robert Moses Plant, near the base of Niagara Falls.
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